Chemical nickel plating of magnesium and its alloys



United States Patent 3,211,578 CHEMICAL NICKEL PLATING 0F MAGNESIUM AND ITS ALLOYS Gregoire Gutzeit, Highland, Ind, assignor to General American Transportation Corporation, Chicago, Ill., a

corporation of New York No Drawing. Filed Nov. 27, 1961, Ser. No. 155,185

8 Claims. (Cl. 117130) The present invention relates to chemical nickel plating of magnesium and its alloys, and more particularly, to processes for effecting such plating with aqueous chemical nickel plating baths of the nickel cation-hypophosphite anion type.

The chemical nickel plating of workpieces having surfaces formed essentially of magnesium and its alloys has not been entirely satisfactory heretofore as such coatings as have been produced thereon readily flake off since the coatings are not intimately bonded thereto. Also such coatings have been characterized by blisters indicating areas therebelow of no adhesion and complete separation of the coatings from the adjacent metal surfaces of the workpieces.

Perhaps the principal reason for this lack of adhesion between the magnesium surface and the coating is that the magnesium surface is highly susceptible to oxidation and films-over prior to or upon contact with the aqueous chemical nickel plating bath; whereby there is a film at the interface between the magnesium surface and the coating of nickel-phosphorus alloy that is inherently produced by a plating bath of this type; which film at the interface prevents proper initiation of the plating and intimate bonding between the magnesium surface and the coating, with the result that the coating is characterized by blisters and other defects evidencing lack of continuity and uniformity of adhesion thereof with respect to the magnesium surface of the workpiece.

Furthermore, the production of desirable coatings upon such magnesium alloy surfaces has been aggravated by the general utilization of a chemical nickel plating bath having a pH in the acid range, as magnesium alloys are inherently sensitive to attach by acid solution even at room temperature, which attack is greatly increased at the normal elevated temperature of operation of the chemical nickel plating bath. The chemical nickel plating bath in the acid range mentioned has been employed heretofore, since such a plating bath in the acid range is quite stable and produces a uniform coating, whereas such a plating bath in the alkaline range is less stable and produces a less desirable coating.

Accordingly, it is a general object of the invention to provide a process and a plating bath for the chemical nickel plating of a workpiece having a metal surface formed essentially of magnesium, wherein the coating produced is intimately bonded to the metal surface and exhibits great adhesion thereto.

Another object of the invention is to provide a process and a plating bath for chemical nickel plating upon a magnesium alloy surface of a workpiece that are productive of a continuous smooth coating of nickel-phophorus alloy that is devoid of blisters and other defects char acterized by lack of adhesion.

Yet another object of the invention is to provide a process of the character noted which employs a chemical nickel plating bath of the nickel cation-hypophosphite anion type of the improved composition which has a pH in the alkaline range and which is stable notwithstanding this circumstance.

Still another object of the invention is to provide an improved chemical nickel plating bath of the nickel cation-hypophosphite anion type and of improved composition which has a pH in the alkaline range and which includes two separate and distinct chelating agents present in amounts sufficient to form mixed chelates with all the nickel ions present in the plating bath.

A further object of the invention is to provide an improved chemical nickel plating bath of the nickel cation-hypophosphite anion type for plating surfaces of magnesium which has a pH in the alkaline range and which has a substantial fluoride content therein in combination with a chelating agent, the plating bath being productive of a continuous, smooth coating that is devoid of blisters and other defects characteristic of lack of adhesion.

Further features of the invention pertain to the particular arrangement of the steps of the process and of the elements of the chemical nickel plating bath, whereby the above-outlined and additional operating features thereof are attained.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof will best be understood as the following specification proceeds.

In accordance with the present invention, there is provided a workpiece or article of manufacture having an outer surface thereof essentially of magnesium or alloy thereof; and ordinarily the workpiece is first machined, or otherwise finished, and thereafter subjected to the process so as to provide on the metal surface thereof a continuous, uniform and smooth coating intimately bonded thereto and formed of a nickel-phosphorus alloy that is inherently produced by chemical deposition from the improved plating bath of the present invention which is of the nickel cation-hypophosphite anion type. Specifically, after the workpiece is machined, or otherwise finished, it is first subjected to pretreatment steps, and then it is subjected to chemical deposition from a plating bath of the type noted having a :pH in the alkaline range to produce a coating intimately bonded to the metal surface and of a smooth and finished character.

Specifically, the present process has been applied in coating the typical magnesium alloys: AZ-31, ZK60A, AZ91, AZ61, AZ92 and AZSO. These typical magnesium alloys have the following composition by weight.

The alloy AZ9 1 is :also identified as AZ-91A and comprises a casting alloy; .the alloy AZ61 corresponds to Dow Alloy K-l and comprises an extrusion alloy; and

i? the alloy AZ*8 1 corresponds to Dow Alloy O-1 and comprises a forging alloy.

In the pretreatment, the workpiece is subjected to the following steps in the order named:

(1) Subject to standard vapor degreasing.

(2) Subject to soaking in an aqueous solution of a suitable cleaner, such as Enthone S160 (about 60 gms. per liter) at an elevated temperature of about 7 C. for a time interval of about mins.

(3) Rinse with water at a temperature of about 10 C.

for a time interval of about 11 min.

(4) Pickle in a 1.0% sulfuric acid solution, (at room temperature for a time interval of about 3 mins.

(5) Rinse with water at a temperature of about 10 C.

for a time interval of about :1 min.

(6) Clean in a solution containing 0.05 mole per liter of trisodium phosphate and 0.05 mole per liter of sodium carbonate at room temperature for a time interval of about 4 rnins.

(7) Rinse with water at a temperature of about 10 C.

for a time interval of ab out 1 min.

After the workpiece has been subjected to pretreatment, as described above, it is transferred to a first preferred chemical nickel plating bath of the nickel cationhypophosphite anion type having a pH in the alkaline range and immersed therein throughout a time interval suificiently long to produce a nickel coating having the desired thickness upon the exterior surface thereof; which plating bath is norm-ally maintained at a relatively high temperature in the general range 90 'C. to 100 C. The plating or coating that is inherently produced by this plating bath essentially comprises about 89% to 97% nickel and about 3% to 11% phosphorus by weight.

This first preferred plating bath essentially comprises an aqueous solution of nickel cations, hypophosphite anions, a chelating agent selected from the class consisting of short chain aliphatic laminocarboxylic acids and salts thereof, fluoride .anions suflicient hydroxyl anions to produce -a pH in the approximate range 7 to 12, and a buffering system comprising alkali metal cations and the anions of the chelating agent. This plating both contains Ni++ and H -PO the absolute concentration of H2PO2 being in the range 0.09 to 1.20 moles per liter, and the ratio between Ni++ and H PO being in the range 0.25 to 1.60; which ratio is productive of a range of Ni++ from 0.02 to 1.92 moles per liter.

This first preferred plating bath has the general composition:

M.p.l. Nifr 0.04 to' 0.17 H2PO2 r00 Aminocarboxylic acid radical 0.08 to 0.40 F 0.05 to 0.50

N-aOI-I to produce pH 7. 0 to'1 2.0.

The preferred composition of this first plating bath is as follows:

M.p.l. Ni+ 0.09 H PO 0.23 Aminocarboxylic acid radical 0:18 P 0.10

NaOH to produce pH 8.5 to 10.5.

Suitable examples of this first preferred plating bath are are follows:

Example] M.p.l. NiSO -'6H O 0.09 NaH PO I-I O Aminoacetic acid (glycine) 0.18 NaF 0.10

NaOH to produce pH 10.5.

Example 2 Nickel hypophosphite 0.09 Hypophosphor-us acid -1 0.07 Aminoacetic acid (glycine) 0.18 Na]? 0.10 NaOH to produce pH 10.5.

Example 3 Nickel sulfate 0.09 Sodium hypophosphite 0.23 Aminosuccinic acid 0.18 Ammonium fluoride 0.10 Sodium fluoride 0.10

NaOH to produce pH 10.5.

In this first preferred plating bath, the Ni++ may be derived from nickel hypophosphite, nickel sulfate, nickel carbonate, etc., and the H PO may be derived from nickel hypophosp hite, hypophosphorous acid, alkali metal hypophosphite, etc. The short chain aliphatic aminocarboxylic acid radical may be derived from either the corresponding acid or the alkali metal salt thereof; this class of acids includes aminoacetic acid (glycine), alpha-aminopropionic acid, beta-aminopropionic acid, alpha-aminobutyric acid, aminosuccinic acid, etc. The F may be derived from hydrofluoric acid, ammonium fluoride, ammonium bifluoride, alkali metal fluoride, etc. The hydroxyl ions may be derived from ammonium hydroxide, or a suitable alkali metal hydroxide, sodium hydroxide being preferred; and any H adjustment of the pH is preferably derived from H The aminocarboxylic acid radical constitutes a chelating agent, which chelating agent is present in an amount to form chelates with a substantial portion and preferably all of the nickel ions in the bath. The F* is employed fundamentally for the purpose of preventing blistering of the nickel coating as it is applied to the magnesium metal surface of the workpiece. A buffering system is also provided comprising the alkali metal cations (Na+) and the aminocarboxylic acid radical. Accordingly, it will be appreciated that the plating bath is buffered so as accurately to maintain the desired pH thereof within the range 8.5 to 10.5 as previously noted.

In the utilization of the first preferred plating bath, nickel cations and hypophosphite anions are depleted; whereby either continuously or periodically the bath is regenerated in use by the addition of these ingredients noted in order to maintain the ranges thereof set forth.

A second preferred plating bath in accordance with the present invention is provided by substituting for the aminocarboxylic acid radical chelating agent of Examples 1 to 3 above a chelating agent selected from the class consisting of short chain aliphatic hydroxycarboxylic acids and salts thereof, the hydroxycar-boxylic acid radical being present in an amount from about 0.04 to 0.20 mole per liter, the preferred concentration being 0.09 mole per liter.

The following are suitable examples of this second preferred plating bath:

Example 4 M.p.l. Nickel sulfate 0.09 Sodium hypophosphite 0.23 Citric acid 0.09 NaF 0.10 NaOH to produce pH 10.5.

Example 5 Nickel sulfate 0.09 Sodium hypophosphite 0.23 Hydroxyacetic acid 0.09 NaF 0.10

NaOH to produce pH 10.5.

The short chain aliphatic hydroxycarboxylic acid radical may be derived from either the corresponding acid or the alkali metal salt thereof; this class of acids includes hydroxyacetic acid, monohydroxysuccinic acid, dihydroxysuccinic acid, gluconic acid, citric acid, hydroxymalonic acid, trihydroxyglutaric acid, alpha-hydroxypropionic acid, beta-hydroxypropionic acid, l-beta-hydroxybutyric acid, etc. In the plating baths of Examples 4 and 5, the hydroxycarboxylic acid radical constitutes a chelating agent and is present in an amount suflicient to form chelates of a substantial amount of the nickel ions in the baths. The plating baths also include a buffering system comprising alkali metal cations (Na*') and the hydroxycarboxylic acid radical.

In a third preferred type of plating bath in accordance with the present invention, two separate and distinct types of chelating agents are utilized so as to form mixed chelates of the nickel ions, and suflicient amounts of the chelating agents are provided to form mixed chelates of all of the nickel ions in the bath. The first chelating agent is selected from the class consisting of short chain aliphatic aminocarboxylic acids and salts thereof including those discussed above with respect to the first preferred plating bath, and the second chelating agent is selected from the class consisting of short chain aliphatic hydroxycarboxylic acids and salts thereof including those discussed above with respect to the second preferred plating bath. The arninocarboxylic acid radical is present in an amount from about 0.08 to 0.40 mole per liter, the preferred concentration being 0.18 mole per liter, and the hydroxycarboxylic acid radical is present in an amount from about 0.04 to 0.20 mole per liter, the preferred concentration being 0.09 mole per liter.

The following are suitable examples of this third preferred plating bath:

Example 6 M.p.l. Nickel sulfate 0.09 Sodium hypophosphite 0.23 Aminoacetic acid 0.18 Citric acid 0.09

NaF 0.10

NaOH to produce pH 10.5.

Example 7 Nickel sulfate 0.09 Sodium hypophosphite 0.23 Aminosuccinic acid 0.09

Hydroxyacetic acid 0.18 NaF 0.10

NaOH to produce pH 10.5.

In the plating baths of Examples 6 and 7, the aminocarboxylic acid radical constitutes a first chelating agent and the hydroxycarboxylic acid radical constitutes a second separate and distinct chelating agent, the two chelating agents named being present in amounts suflicient to form mixed chelates of all of the nickel ions present in the baths. It has been found that the plating baths containing the mixed chelating agents is a substantial improvement over those baths using only one of the chelating agents comprising the mixed chelating system. The plating baths also include a buffering system comprising alkali metal cations (Na+) and the cations of the two chelating agents present, namely, the aminocarboxylic acid radical and the hydroxycarboxylic acid radical.

The performance of any of the above described plating baths set forth in Examples 1 through 7 is improved by the addition of an exalting agent thereto, and also be the incorporation therein of additional sodium cations which may be derived from any of the common salts thereof such as sodium sulfate. The exalting agent is selected from the class consisting of simple short chain saturated dicarboxylic acids and salts thereof, the dicarboxylic acid radical being present in an amount from about 0.01 to 0.10 mole per liter, the preferred concentration being 0.06 mole per liter.

The following are suitable examples of plating baths incorporating therein exalting agents:

Example 8 M.p.l. Nickel sulfate 0.09 Sodium hypophosphite 0.225

Sodium hypophosphite 0.225

Aminoacetic acid (glycine) 0.18 Sodium citrate 0.09 Sodium succinate 0.06

NaF 0.10 Sodium sulfate 0.09 NaOH to produce pH 10.53.

Example 11 Nickel sulfate 0.09

Sodium hypophosphite 0.225

Aminoacetic acid (glycine) 0.09 Sodium citrate 0.18

Sodium succinate 0.06

NaF 0.10

Sodium sulfate 0.09

NaOH to produce pH 10.52.

The simple short chain saturated dicarboxylic acid radical used as the exalting agent is a type of compound separate and distinct from those types of compounds utilized as the mixed chelating agents and the exalting agent may be derived from either the corresponding acid or the alkali metal salt thereof; this class of acids includes malonic acid, succinic acid, glutaric acid, adipic acid, etc. In the plating baths of Examples 8 to 11, the dicarboxylic acid radical constitutes an exalting agent and is present in amounts sufficient substantially to exalt the plating rate of the plating baths. The plating baths also include an auxiliary buffering system comprising alkali metal cations (Na+) and dicarboxylic acid radical, which auxiliary buffering system supplements the buffering action of the fundamental buffering system comprising alkali metal cations (Na+) and the aminocarboxylic acid radical and the hydroxycarboxylic acid radical.

In passing it is noted that all of the plating baths of the present invention are substantially free of chloride ion, since it has been found that the chloride ion is detrimental to the action of the plating baths on surfaces comprising magnesium and its alloys, whereas plating baths containing sulfate ions, as the anion of the nickel compound in the bath and as the anion of any additional sodium ions needed in the bath, does not interfere with the operation of the plating baths in plating metal surfaces comprising magnesium and alloys thereof.

In the utilization of any of the above plating baths, nickel cations and hypophosphite anions are depleted, whereby continuously or periodically the baths are regenerated in use by the addition of these ingredients noted in order to maintain the ranges thereof set forth, all as has been explained above with respect to the first preferred plating bath of this invention.

When the present process is carried out in the manner described utilizing the improved plating baths of the present invention, smooth, bright coatings are obtained on metal surfaces comprising essentially magnesium and alloys thereof, the coatings being entirely devoid of blisters or other defects characteristic of lack of adhesion and being intimately bonded to the workpieces.

In view of the foregoing, it is apparent that there have been provided improved processes and improved plating baths for producing coatings of nickel-phosphorus alloys on surfaces of workpieces formed of magnesium and its alloys which coatings are smooth, bright, uniform, continuous and intimately bonded to the workpieces.

While there has been described what are at present considered to be the preferred embodiments of the invention, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope ofthe invention.

What is claimed is:

1. An aqueous chemical nickel plating bath comprising nickel ions, hypophosphite ions in the range 0.09 to 1.20 moles per liter, the ratio between nickel ions and hypophosphite ions being in the range 0.25 to 1.60,

a first chelating agent selected from the class consisting of short chain aliphatic aminocarboxylic acids and salts thereof and in the range 0.08 to 0.40 mole per liter, a second chelating agent selected from the class consisting of short chain aliphatic hydroxycarboxylic acids and salts thereof and in the range 0.04 to 0.20 moleperliter, said two chelating agents named forming mixed chelates of all of the nickel ions in said bath, fluoride ions in the range 0.10 to 0.50 mole per liter, an exalting agent selected from the class consisting of simple short chain saturated dicarboxylic acids and salts thereof, said exalting agent being present in an amount sufficient substantially to increase the plating rate of said bath, sufficient hydroxyl ions to produce a pH in the approximate range 7 to 12, and a buffering system comprising alkali metal cations and the anions of said two chelating agents named.

2. An aqueous chemical nickel plating bath comprising nickel ions, hypophosphite ions in the range0.09 to 1.20 moles per liter, the ratio between nickel ions and hypophosphite ions being in the range 0.25 to 1.60, a first chelating agent selected from the class consisting of short chain aliphatic aminocarboxylic acids and salts thereof and in the range 0.08 to 0.40 mole per liter, a second chelating agent selected frorn'the class consisting of short chain aliphatic hydroxycarboxylic acids and salts thereof and in the range 0.04 to 0.20 mole per liter, said two chelating agents named forming mixed chelates of all of the nickel ions in said bath, fluoride ions in the range 0.10 to 0.50 mole per liter, sufiicient hydroxyl ions to produce a pH in the approximate range 7 to 12, and a buffering system comprising alkali metal cations and the anions of said two chelating agents named.

3. The plating bath set forth in claim 2, wherein said first chelating agent is selected from the class consisting of aminoacetic acid and the alkali metal salts thereof.

4. The plating bath set forth in claim 2, wherein said second chelating agent is selected from the class consisting of citric acid and the alkali metal saltsthereof.

5. An aqueous chemical nickel plating bath comprising nickel ions, hypophosphite ions in the range 0.09 to 1.20 moles per liter, the ratio between nickel ions and hypophos hite ions being in the range 0.25 to 1.60, a first chelating agent selected from the class consisting of aminoacetic acid and the alkali metal salts thereof and in the range 0.08 to 0.40 mole per liter, a second chelating agent selected from the class consisting of citric acid and the alkali metal salts thereof and in the range 0.04 to 0.20 mole per liter, said two chelating agents named forming mixed chelates of all of the nickel ions in said bath, fluoride ions in the range 0.10 to 0.50 mole per liter, sufiicient hydroxyl ions to produce a pH in the approximate range 7 to 12, and a buffering system comprising alkali metal cations and the anions of said two chelating agents named.

6. The plating bath set forth in claim 5, wherein the absolute content of said first chelating agent is approximately 0.18 mole per liter, and the absolute content of said second chelating agent is approximately 0.09 mole per liter.

7. The process of plating with nickel the metal surface of a workpiece, wherein said metal surface is formed essentially of magnesium; said process comprising cleaning and pickling said metal surface, and then consisting said metal surface with a hot aqueous chemical nickel plating bath during a time interval sufficiently long to produce a nickel coating of desired thickness upon said metal surface; wherein said aqueous chemical nickel plating bath comprises the composition set forth in claim 1.

8. The process of plating with nickel the metal surface of a workpiece, wherein said metal surface isformed essentially of magnesium; said process comprising cleaning and pickling said metal surface, and then contacting said metal surface with a hot aqueous chemical nickel plating bath during a time interval sufliciently long to produce a nickel'coating of desired thickness upon said metal surface; wherein said aqueous chemical nickel plating bath comprises nickel ions, hypophosphite ions in the'range 0.09 to 1.20 moles per liter, the ratio between nickel ions and hypophosphite ions being in the range 0.25 to 1.60, a first chelating agent selected from the class consisting of short chain aliphatic aminocarboxylic acids and salts thereof and in the range 0.08 to 0.40 mole per liter, a second chelating agent selected from the class consisting of short chain aliphatic hydroxycarboxylic acids and salts thereof and in the range 0.04 to 0.20 mole per liter, said two chelating agents named forming mixed chelates of all of the nickel ions in said bath, fluoride ions in the range 0.10 to 0.50 mole per liter, sufiicient hydroxyl ions to produce a pH in the approximate range 7 to 12, and a buffering system comprising alkali metal cations and the anions of said two chelating agents named.

References Cited by the Examiner UNITED STATES PATENTS 2,694,019 11/54 Gutzeit 1l7130 2,819,187 1/58 Gutzeit et a1 1l7130 2,822,293 2/58 Gutzeit et al. 117-l30 RICHARD D. NEVIUS, Primary Examiner. JOSEPH B. SPENCER, Examiner. 

8. THE PROCESS OF PLATING WITH NICKEL THE METAL SURFACE OF A WORKPIECE, WHEREIN SAID METAL SURFACE IS FORMED ESSENTIALLY OF MAGNESIUM; SAID PROCESS COMPRISING CLEANING AND PICKLING SAID METAL SURFACE, AND THEN CONTACTING SAID METAL SURFACE WITH A HOT AQUEOUS CHEMICAL NICKEL PLATING BATH DURING A TIME INTERVAL SUFFICIENTLY LONG TO PRODUCE A NICKEL COATING OF DESIRED THICKNESS UPON SAID METAL SURFACE; WHEREIN SAID AQUEOUS CHAMICAL NICKEL PLATING BATH COMPRISES NICKEL IONS, HYPOPHOSPHITE IONS IN THE RANGE 0.09 TO 1.20 MLES PER LITER, THE RATIO BETWEEN NICKEL IONS AND HYPOPHOSPHITE IONS BEING IN THE RANGE 0.25 TO 1.60, A FIRST CHELATING AGENT SELECTED FROM THE CLASS CONSISTING OF SHORT CHAIN ALIPHATIC AMINOCARBOXYLIC ACIDS AND SALTS THEREOF AND IN THE RANGE OF 0.08 TO 0.40 MOLE PER LITER, A SECOND CHELATING AGENT SELECTED FROM THE CLASS CONSISTING OF SHORT CHAIN ALIPHATIC HYDROXYCARBOXYLIC ACIDS AND SALTS THEREOF AND IN THE RANGE OF 0.04 TO 0.20 MOLE PER LITER, SAID TWO CHELATING AGENTS NAMED FORMING MIXED CHELATES OF ALL OF THE NICKEL IONS IN SAID BATH, FLUORIDE IONS IN THE RANGE 0.10 TO 0.50 MOLE PER LITER, SUFFICIENT HYDROXYL IONS TO PRODUCE A PH IN THE APPROXIMATE RANGE 7 TO 12, A BUFFERING SYSTEM COMPRISING ALKALI METAL CATIONS AND THE ANIONS OF SAID TWO CHELATING AGENTS NAMED. 